Common disinfection methods for water, for example swimming pool water, include a dosing of a disinfectant, commonly a chlorine disinfectant, into the water.
Conventionally the chlorine disinfectant is added to water, e.g. swimming pool water, as chlorine gas, sodium- or calcium-hypochlorite while these reactants dissolve in the water to free chlorine species (free chlorine), mainly hypochloric acid (HOCl) and a hypochlorite ion (OCl−) at pH dependent ratios.
Chlorine disinfectants are commonly accepted to be a easy to handle and economic disinfection solutions in water treatment.
While inactivating pathogen microorganisms, the chlorine disinfectants can react with other constitutes of a water matrix in pools and in this way organic and inorganic disinfection by products (DBPs) are formed. DBPs are mainly generated due to the reaction of chlorine with impurities mainly introduced by swimmers but as well due to chlorine decay over time or/and in the presence of UV irradiance.
DBPs have negative health impacts for water consumers or swimmers being exposed to these DBPs, e.g. irritating eyes and respiration of swimmers.
Organic DBPs in pools include chloramines (mono-, di- and tri-chlorinated), referred to as combined chlorine, and trihalomethanes (THMs), maily chloroforms. THMs occur due to a reaction of organic water constitutes, e.g. citric acid introduced by bathers, in contact with chlorine. Their level depends on the THM-formation potential of the organic substances in the water, chlorine concentration, pH, temperature and exposure time.
Main inorganic DBPs are chlorite (ClO2-) and chlorate (ClO3-) being decay-products of free chlorine while their concentration depends on an organic load of a disinfected water, temperature, exposure time, chlorine concentration, chlorine storage time before use and UV irradiance.
As DBPs have said negative health impacts for water consumers or swimmers a reduction of the DBPS by a water treatment will be vital for a high water quality.
The DBP combined chlorine is often reduced photolytically by UV light. In outdoor pools sunlight most often reduces combined chlorine under restricted levels and in indoor pools artificial UV light is used most often in the form of low or medium pressure UV devices.
The inorganic DBP chlorite could also be eliminated by UV, but chlorate not.
Combined chlorine and THMs can be reduced by their adsorption on activated carbon. Inorganic by products do not adsorb on this solid and therefore their concentration accumulates over time. Besides this treatment has a high cost.
Filtration by reverse osmosis (RO) allows the reduction of all resumed water pollutants, producing deionized water. This water needs to be post treated with salt solutions to obtain the required properties for its use in swimming pools. The total costs of RO treatment are too high for most public pools.
It is further known that chlorine disinfectants are very instable to UV irradiation—being degraded by said UV irradiation (photolytic chlorine degradation) (Mariko Tachikawa et al. “Effects of Isocyanuric Acid on the Monochlorodimedone Chlorinating rates with Free Chlorine and Ammonia Chloramine in Water”, Water Research, 36 (2002), pp. 2547-2554; J. Gardiner, “Choloisocyanurates in the Treatment of Swimming Pool Water”, Water Research Pergamon Press 1973, Vol. 7, pp. 823-833).
Especially during “open” water chlorine disinfection, particularly outdoor swimming pool water chlorine disinfection, said chlorine disinfectant dosed/contained in said “open” water, i.e. said outdoor swimming pool water, will strongly be degraded by sunlight (photolytic chlorine degradation).
An effect of photolytic chlorine degradation will increase for regions of high solar irradiance, for example southern European countries, but can also be relevant and/or significant—on sunny days—in other regions, such as northern European countries.
Photolytic chlorine degradation by water chlorine disinfection can be balanced by an increased chlorine disinfectant dosing, to maintain same, i.e. consistent effective, chlorine disinfectant concentration in said water to disinfect.
This effort will increase process costs while being unsatisfying for an operator/customer. Besides, the DBPs increase, i.e. accumulate, due to increased chlorine consumption.
Another option can be using stabilized chlorine (stabilized chlorine disinfectant) as a chlorine disinfectant with a stabilizing compound (chlorine stabilizer) stabilizing said chlorine disinfectant, i.e. improving an UV stability of said chlorine disinfectant.
The most successful of this stabilizing compound in use at present is isocyanuric acid (ICA), which forms reversibly 1-3-chlorinated chloroisocyanurates with the chlorine disinfectant in aqueous solution, i.e. in said water, and stabilizing said chlorine disinfectant (Mariko Tachikawa et al. “Effects of Isocyanuric Acid on the Monochlorodimedone Chlorinating rates with Free Chlorine and Ammonia Chloramine in Water”, Water Research, 36 (2002), pp. 2547-2554; J. Gardiner, “Choloisocyanurates in the Treatment of Swimming Pool Water”, Water Research Pergamon Press 1973, Vol. 7, pp. 823-833; C. J. Downes et al., “Determination of Cyanuric Acid Levels in Swimming Pool Waters By u.v. Absorbance, HPLC and Melamine Cyanurate Precipitation”, Water Research Vol. 18, No. 3, pp. 277-280, 1984).
This stabilized chlorine disinfection of the (swimming pool) water can be conveniently carried out with chlorinated isocyanuric acid, for example solid chloroisocyanurate (tablets), usually a sodium or potassium salt of a dichloro-compound, while said chlorinated isocyanurate species forming mono-, di- and tri-chlorinated species in the presence of said chlorine disinfectant dissolved in water, i.e. of free chlorine (J. Gardiner, “Choloisocyanurates in the Treatment of Swimming Pool Water”, Water Research Pergamon Press 1973, Vol. 7, pp. 823-833; C. J. Downes et al., “Determination of Cyanuric Acid Levels in Swimming Pool Waters By u.v. Absorbance, HPLC and Melamine Cyanurate Precipitation”, Water Research Vol. 18, No. 3, pp. 277-280, 1984).
An inconvenience for the use of chlorinated isocyanuric acid instead of non-stabilized chlorine disinfection by free chlorine is the decrease in disinfection safety at increasing isocyanuric acid concentrations (Golaszweski G., Seux R., “The kinetics of the action of chloroisocyanurates on three bacteria: pseudomonas aeruginosa, streptococcus faecalis and staphylococcus aureus”, Wat. Res. 28.1, 207-217, 1994) while said isocyanuric acid also having negative health impacts for water consumers or swimmers being exposed.
The available free chlorine of the chlorinated isocyanuric acid, responsible for disinfection is consumed over time due to the chlorine demand of the swimming pool matrix. As a consequence more chlorinated isocyanuric acid is added to the pool and isocyanuric acid, the solution product of chlorinated isocyanuric acid, accumulates to unsafe concentrations, e.g. up to 140 mg/l and higher.
As it is the case for other applications, pool water disinfection is always fighting to find a best balance between microbiological safety and chemical impact of disinfectants in the water—on human and ecological environment.
Known (concentration) measurement methods for said chlorine disinfectant and said isocyanuric acid are spectrometric methods (DPD, Melanin based CyA-Test).
Equipments for dosing as well as for a controlled dosing of chlorine disinfectant to water to be treated are known as well as equipments for irradiating water with UV, for example Wallace & Tiernan®, “Wasseraufbereitungs—und Desinfektionssysteme”, Oktober 2010.
(Online) Sensors measuring chlorine disinfectant concentrations of chlorine disinfectant contained in water are known, for example a membrane sensor FCl (Free Chlorine) or TCl (Total Chlorine) as well as a bare electrode sensor Depolox5 of Wallace & Tiernan (Wallace & Tiernan, Siemens, Water Technologies, Produktinformation zu Membransensor FCl, TCl und zu Depolox5 Sensor).